1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a CPP sensor with improved ferromagnetic layers.
2. Background of the Invention
One type of conventional magnetoresistive sensor used as the read head in magnetic recording disk drives is a “spin-valve” (SV) sensor. A SV magnetoresistive sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance.
In a magnetic recording disk drive SV read sensor or head, the magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance. If the sense current flowing through the SV is directed parallel to the planes of the layers in the sensor stack, the sensor is referred to as a current-in-the-plane (CIP) sensor, while if the sense current is directed perpendicular to the planes of the layers in the sensor stack, it is referred to as current-perpendicular-to-the-plane (CPP) sensor. CPP-SV read heads are described by A. Tanaka et al., “Spin-valve heads in the current-perpendicular-to-plane mode for ultrahigh-density recording”, IEEE TRANSACTIONS ON MAGNETICS, 38 (1): 84-88 Part 1 January 2002.
The fixed or pinned ferromagnetic layer in a CPP-SV sensor used in read heads may be an antiparallel (AP) pinned structure. The AP-pinned structure has first (AP1) and second (AP2) ferromagnetic layers separated by a nonmagnetic antiparallel coupling (APC) layer with the magnetization directions of the two AP-pinned ferromagnetic layers oriented substantially antiparallel. The AP2 layer, which is in contact with the nonmagnetic APC layer on one side and the sensor's spacer layer on the other side, is typically referred to as the reference layer. The AP1 layer, which is typically in contact with an antiferromagnetic or hard magnet pinning layer on one side and the nonmagnetic APC layer on the other side, is typically referred to as the pinned layer. If the AP-pinned structure is the “self-pinned” type, then no pinning layer is required. In a self-pinned structure where no antiferromagnet or hard magnet pinning layer is present, the AP1 layer is in contact with a seed layer on the sensor substrate. The AP-pinned structure minimizes magnetostatic coupling between the reference layer and the CPP-SV free ferromagnetic layer. The AP-pinned structure, also called a “laminated” pinned layer, and sometimes called a synthetic antiferromagnet (SAF), is described in U.S. Pat. No. 5,465,185.
In CPP-SV sensors, because the sense current flows perpendicular to all the layers in the sensor stack, the electrical resistance of the active region (the free layer, spacer layer and the reference layer of the AP-pinned structure) is a relatively small part of the total resistance of the sensor. Antiferromagnetic pinning layers such as PtMn or IrMn do not contribute to the magnetoresistance signal; rather they constitute a parasitic resistance that decreases the overall signal significantly since its magnitude can be similar or greater than the total resistance of the active part of the spin-valve structure. This parasitic resistance is large compared to a typical magnetoresistance resistance-area product (ΔRA) originating from the active region. It is thus desirable to increase the resistance of the active region without significantly increasing the total stack resistance.
The materials making up the free layer and the reference layer in an AP-pinned structure are typically crystalline alloys of CoFe or NiFe. These materials have a relatively low electrical resistivity (ρ) and thus do not contribute significantly to the magnetoresistance ΔRA of the active region. Moreover, it is known that an increase in ρ in the materials making up the free layer and the reference is associated with a shortening of the spin diffusion length (SDL). It is desirable to shorten SDL to the extent that it becomes comparable to the thickness of the magnetic layers in the active region, which means that more of the active region is available for bulk electron scattering. This results in an increase in the magnetoresistance (ΔR/R) of the CPP read head. For example, the SDL for crystalline CoFe alloys is greater than the typical thickness of the AP2 layer, which is about 15-30 Å, so that not all of the thickness of the AP2 layer is being used for bulk electron scattering.
What is needed is a CPP-SV sensor that has an AP-pinned structure and that has improved ferromagnetic materials for the free layer and the reference layer that increase the magnetoresistance of the sensor.